A substantial amount of world energy production is by combustion of fossil fuels. As global energy demands relentlessly increase, fossil fuels are becoming exhausted and are also implicated in the progressively increasing concentration of carbon dioxide in the earth's atmosphere, which according to the intergovernmental panel on climate change (IPCC) predictions, will reach the critical limit by 2030 if we continue to do business as usual. Hence, a key challenge facing world industrial economies is the development of environmentally benign, renewable energy. A significant technology currently under intense development for producing renewable energy is photovoltaic (PV) cells, which directly convert incident light into electricity. The produced electrical power can be used for driving any of various electrical loads such as, for example, lighting, heating, or operation of electronic equipment. Other uses include charging of batteries or other energy storage devices.
PV cells produce direct-current electricity by the photovoltaic (PV) effect, in which a voltage or electrical current is produced in a photoconductive material by exposing the material to light of one or more particular wavelengths. The PV effect is different from the photoelectric (PE) effect. Whereas the PE effect involves ejection of electrons from a material that is exposed under certain conditions to light, the PV effect involves light-induced shifting of electrons of a photoconductive material from lower to higher energy bands (e.g., from a valence band to a conduction band) when the material is illuminated by light of a certain wavelength(s). As discussed more fully below, this electron shifting produces a potential difference across electrodes in contact with the photoconductive material and can produce sufficient electrical current to drive a load. Substantially all PV devices are like photodiodes, wherein the term “photovoltaic” denotes the unbiased operating mode of a photodiode, wherein current through the photodiode is produced entirely by incident light energy.
Most PV cells currently in use are made of a rigid, inorganic semiconductor material such as crystalline silicon, gallium arsenide, copper indium selenide, or cadmium telluride. Unfortunately, these cells and panels thereof are expensive, particularly as a result of the complexity of processes for purifying and converting raw materials for use in these cells. For example, solar-grade silicon is very high purity, which is expensive to produce. Also, crystalline materials, especially having a large surface area, are difficult to produce due especially to problems associated with producing large crystals that do not have a significant density of efficiency-degrading defects. These materials also raise concerns about their disposal in a responsible manner, especially on a large scale.
More recent approaches to making PV cells stemmed from discoveries of the photovoltaic behavior of certain organic molecules, including certain polymers and small-molecule chromophores, called “conjugated” materials. The molecular structure of a conjugated material includes alternating single and double bonds of adjacent carbon atoms; e.g., a conjugated organic polymer has a carbon backbone comprising alternating single and double covalent bonds of the carbon atoms. Compared to conventional semiconductors such as silicon, conjugated compounds are relatively easy to produce and incorporate into products. Many are mechanically flexible and are relatively easy to dispose of in an environmentally sensible manner. Compared to silicon, these materials have low mass and can be produced at substantially lower cost. Also, many of these compounds are relatively easily processed in a manner useful for making PV devices, such as forming them into films using conventional techniques.
An organic PV cell generally functions by undergoing the following four physical processes: (a) absorption of light, (b) diffusion of the excited state (“exciton”) to the heterojunction, (c) light-induced charge-transfer (i.e., separation of opposite charges) and charge transport toward respective electrodes, and (d) charge collection at the electrodes. To such end, a typical organic PV cell comprises a “photoactive layer” that includes a first substance termed an “electron donor” or simply “donor” and a second substance termed an “electron acceptor” or simply “acceptor.” The conjugated material is normally the electron donor (also called “electron absorber” or simply “absorber”). The electron donor is the material that absorbs incident photons (having a wavelength absorbable by the material). In the absorber, absorbed photons produce charge pairs (elevated-energy electrons and corresponding “holes”) in the material. The acceptor typically has higher electron affinity (“EA”) than the donor. Fullerene (C60) is often used as an acceptor, especially in planar heterojunction organic PV devices, due to its high electron affinity and its ability to be vapor-deposited.
The donor (D) functions essentially as a p-type semiconductor, and the acceptor (A) functions essentially as an n-type semiconductor. More specifically, the donor functions as an “electron-ejecting” material upon absorption of photon, and the acceptor functions as an “ejected-electron accepting” material. Upon encountering an incident photon (hv) of proper wavelength, the following photo-excitation reaction occurs with the donor and acceptor:D+A+hv→D*+A(or D+A*)→D.++A.−in which D* and A* are the excited state of the donor and acceptor, respectively. Photo-excitation is followed by formation of this charge-separated state, consisting of the radical cation of the donor (D.+) and the radical anion of the acceptor (D.−).
Conjugated materials useful as electron donors have multiple delocalized π electrons that are normally produced by hybridization of carbon p-orbitals in the material's conjugated molecular structure. When excited by an absorbed photon having a particular wavelength, a π electron is excited to delocalize from a highest occupied molecular orbital (HOMO) to a lowest unoccupied molecular orbital (LUMO). This delocalization jump is called a π-π* transition, in which π denotes the bonding orbital (HOMO) of the electron, and π* denotes a corresponding anti-bonding orbital (LUMO) of the electron. (The hole is regarded as being in the HOMO.) The energy “bandgap” is the separation between the LUMO and HOMO, which is related to the particular absorbed wavelength of light. The electron experiencing the π-π* transition produces a corresponding “hole,” and the electron and hole are collectively termed an “electron-hole pair.”
An “exciton” in a photo-active material is an electron-hole pair in a bound state. An exciton has a defined lifetime before undergoing geminate recombination, i.e., a process in which the original electron and hole recombine with each other rather than recombining with holes or electrons from other pairs. To produce a photo-current the electron and hole of a given exciton must separate from each other. Otherwise, they may recombine. For the material to produce a useful electrical current, the electron and hole must be collected separately at respective electrodes before they can recombine.
In most organic PV devices (“OPV” devices) the donor and acceptor materials are sufficiently dissimilar that they are at least partially non-miscible with each other. As a result, in an OPV device, respective units of each material contact each other at one or more “heterojunctions” that are effectively p-n interfaces. The (singlet) excitons diffuse through the donor layer toward the heterojunction via Forster energy transfer. At the interface, the electrons become separated from the holes, a process called “exciton dissociation.” Exciton dissociation also results in the electron energy dropping from the conduction band of the donor to the conduction band of the acceptor. (These conduction bands have respective edges, and the band edge of the acceptor should be lower than the band edge of the donor to ensure proper charge migration.) From the dissociated exciton and if the charge-carrier mobility of the active material is sufficient, the electron and hole (as respective “charge carriers”) are collected at respective electrodes of the OPV device. If the charge-carrier mobility of the active material is too low in view of the mean distance in the material to a p-n interface, the charge carriers do not reach the electrodes, instead undergoing recombination (via intrinsic radiative and non-radiative decay processes), for example, or remaining uncombined and possibly interfering with migration of other charge carriers in the cell.
Conventional OPV devices (also called “OPV cells”) have one of two general structural configurations. One configuration is termed a “planar heterojunction,” in which a layer of photoactive material (comprising a layer of the donor and a layer of the acceptor) is sandwiched between the electrodes in a planarly laminar configuration. One of the electrodes is transparent to at least certain useful wavelengths of incident light (especially the wavelength(s) that produce excitons in the donor material), and the other electrode usually is reflective to the incident light. The interface between the donor and acceptor layers constitutes the junction, which is called a “heterojunction” because the donor and acceptor are different materials. Desirably, excitons created in the donor layer diffuse to the heterojunction, where the charges separate from each other, with the hole remaining in the donor and the electron passing into the acceptor on its way to a respective electrode.
Planar heterojunctions are easy to form but tend be inefficient. Excitons typically have diffusion lengths of approximately 3 to 10 nm in a photoactive material. This requires that the donor and acceptor layers be very thin to facilitate successful diffusion of charges to the electrodes. Generally, the thinner the photoactive material, the less light it can absorb. The less light that is absorbed, the fewer excitons that are produced, and the lower the efficiency of the cell. Thicker layers do not absorb significantly more light than a thinner film, but they do exhibit a large series resistance.
The second structural configuration is termed a “bulk heterojunction” (“BHJ”), in which the layer of photoactive material is a mixture of the donor and acceptor materials. To form a BHJ, the donor and acceptor materials should be immiscible, and when they are mixed together they tend to phase-separate from each other. Appropriate agitation during mixing can produce a donor-acceptor mixture in which very small bits (in the 1-100 nanometer range) of each material are uniformly distributed throughout the x, y, and z dimensions of the mixture. This mixture forms a corresponding distribution of very small p-n junctions throughout the “bulk” of the photoactive material. Desirably, the bits of donor and acceptor have a mean separation from one another by distances in the range of approximately 5-10 nm (the usual range of diffusion length of the excitons) to increase the probability of successful charge diffusion to the heterojunctions and correspondingly to reduce the probability of carrier recombination. Thus, although BHJs are less limited in terms of active-material thickness, the performance of conventional BHJ cells is affected by many variables that are difficult to control or maintain at a consistent level.
Basically, the overall efficiency of an OPV cell is the ratio of electrical power the device can deliver to a load, relative to the light power incident on the device. Efficiency is expressed in several different ways. The “quantum efficiency” (“QE”) of the cell is the ratio of the number of charge carriers (excitons) produced by the cell to the number of photons of a particular wavelength (and thus of a particular energy) incident on the cell. For example, if all the incident photons of a certain wavelength are absorbed and converted into respective excitons, then the QE of the cell for the wavelength would be unity. This is an ideal situation that is not met with current PV technology because of efficiency-robbing phenomena normally occurring in the cell such as: (a) short exciton lifetime and diffusion length, (b) geminate and bimolecular recombinations of excitons before they reach the heterojunctions, (c) lack of precise control over the morphology of the active layer, (d) poor mobility of charge carriers, and (e) reflection and scattering of incident light. QE is of two types: external quantum efficiency (EQE) and internal quantum efficiency (IQE). EQE is a ratio of the number of charge carriers produced and collected by the OPV cell to the number of photons of a given wavelength incident on the cell. IQE is the ratio of the number of charge carriers produced and collected by the PV cell to the number of photons incident on the cell and absorbed by the cell. IQE is always greater than EQE.
A monochromatic version of EQE is called “Incident Photon to Electron-Conversion Efficiency” or “Incident Photon to Current Efficiency” (abbreviated “IPCE”). IPQE is the ratio of photons actually producing electrons that are delivered by the cell to a load, relative to photons of a particular monochromatic wavelength of light incident on the cell. A monochromatic version of IQE is the “Absorbed Photon to Current Efficiency” (abbreviated “APCE”), which is the ratio of photons actually producing electrons delivered by the cell to a load, relative to photons of a particular monochromatic wavelength light actually absorbed by the cell.
Another expression of efficiency is the cell's energy-conversion efficiency (η), which is the percentage of power converted (from absorbed light to electrical energy) and collected, when the cell is connected to an electrical load. This term may be calculated using the ratio of the maximum power point (Pm) to the incident light irradiance (E, in W/m2) under standard test conditions (“STC”), and the surface area of the cell (Ac, in m2):
  η  =            P      m              E      ×              A        c            STC specifies a temperature of 25° C. and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum, which corresponds to the irradiance and spectrum of sunlight incident on a clear day on a sun-facing 37° tilted surface with the sun at an angle of 41.81° above the horizon.
Thus, the efficiency of organic PV cells is limited by the number of photons that can be absorbed within the thickness of the layer of photoactive material. For most chromophores, absorption is confined to the visible region of the electromagnetic spectrum; meanwhile, approximately 50% of the AM1.5G solar irradiance is in the near-IR region. The best organic photovoltaic OPV devices currently available are based on active materials comprising poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM), which is transparent in the near-IR region. As a result, substantially none of the near-IR radiance is captured by the cell and used to produce electricity. If the absorption of thin layers of photoactive material could be extended to the near-IR with no significant loss in Voc, a significant improvement in power-conversion efficiency (η) would be possible. (Voc is “photovoltage at open circuit,” which is the voltage output from the PV cell being irradiated but not connected to a load.)
Various soluble trivalent- and tetravalent-metal-substituted phthalocyanines (“MPcs”, wherein M=AlCl, GaCl, InCl, or V═O) are known structurally and for various uses such as optical limiting devices and donor layers in organic photovoltaics (“OPVs”). Trivalent and tetravalent metal phthalocyanines exhibit higher photoactivity, ionization potentials, charge-generation efficiency, and non-linear susceptibility compared to divalent-metal phthalocyanines (e.g., CuPc), making the trivalent and tetravalent Pcs better candidates for use in OPV devices. The presence of a dipole in the axial direction in these MPcs, perpendicular to the molecular plane, assists the formation of various polymorphs, some of which being photosensitive in the near-IR portion of the electromagnetic spectrum. Polymorphism is the ability of certain molecules to crystallize into different structural forms (unit cells). For example, thin films of TiOPc have been made that include any of several crystalline polymorphs of the compound. But, this compound has not been made soluble so that thin films could be formed of it using solvent-processing techniques.
Poor solubility of trivalent and tetravalent metal phthalocyanines in common organic solvents necessitates: (a) purification by non-ideal methods such as entrainer sublimation and (b) processing by expensive vapor-deposition. Hence, soluble MPc derivatives are needed that can be purified using column-chromatography and processed into thin-films using techniques such as reel-to-reel wet-coating and ink-jet printing. In this context, successful results from attempts to obtain MPc polymorphs from solution-processed films have been elusive. For example, as reported in the literature, spin-coated films of t-butyl-substituted TiOPc derivatives do not lead to near-IR active polymorphs. The TiOPc derivatives reported hereinbelow lend themselves into polymorphs with tunable near-IR sensitivity when layered (e.g., by spin-coating), as a solution in a common organic solvent, on a selected substrate. However, to the best of Applicants' knowledge, these soluble derivatives have not heretofore been used as electron donors for organic PV devices.
In view of the foregoing, there remains a need for organic PV devices providing greater efficiency (including ability to absorb near-IR light) and that can be fabricated by solution-processing on any of various substrates, including rigid and flexible substrates.